The present invention relates to a hydrodynamic brake and particularly to a brake which moves fluid rapidly into and out of the brake.
Auxiliary brakes are principally used in heavy vehicles, such as lorries and buses, in order to avoid wear of the ordinary brakes of the vehicle, such as the service brakes, for example at braking on long downhill slopes. An example of such an auxiliary brake is a hydrodynamic retarder, which generates a braking moment by means of any suitable oil between a stator and a rotor. The stator and the rotor constitute together a toroid-shaped space, which is called torus. The stator as well as the rotor is provided with a plurality of vanes and the oil is guided during the rotation of the rotor by the vanes within the toroid-shaped space. During the braking process, the kinetic energy of the oil is transformed to heat energy. The oil has to be cooled after it has left the toroid-shaped space. Thereafter the oil may again be supplied to the toroid-shaped space. The retarder is connected to the power train of the vehicle, for example, at the propeller shaft in connection to the gearbox of the vehicle for allowing braking of the driving wheels of the vehicle. The retarder may be provided directly on the propeller shaft or be connected to that shaft via a gear unit. At a low number of revolutions of the propeller shaft and when the space between the stator and the rotor, i.e. the toroid-shaped space, is filled with oil, the braking effect of the retarder increases with the rate of rotation of the propeller shaft, and the output braking moment of the retarder is substantially proportional to the number of revolutions of the propeller shaft. Consequently, at a certain number of revolutions of the propeller shaft, a certain maximum output braking moment may be obtained. At higher number of revolutions of the propeller shaft, the output braking moment depends on the coefficient of fullness of oil in the toroid-shaped space. Consequently, by controlling the pressure in the space, the output braking moment may be adjusted. Before the retarder begins to produce an output braking moment, the space between the stator and the rotor has to be filled with oil. Previously, the retarder has comprised a pump and an oil reservoir. The lower the speed of the propeller shaft of the vehicle is, the longer time it take to fill the space around the stator and the rotor. In order to speed up this operation, an oil accumulator is often used in connection with the oil reservoir.
The working medium of a hydrodynamic retarder is any suitable oil. As mentioned above, the kinetic energy of the oil is transformed to heat energy during a braking process. In order to avoid overheating of the oil, it is important that the oil flow through the toroid-shaped space be high. With a high oil flow, one may obtain an effective cooling of the oil. In order to obtain a high flow of oil, the pressure differences, which are created when the rotor rotates in a certain direction of rotation, are used. After the oil has left the toroid-shaped space, the oil is guided to a cooling system/heat exchanger of the vehicle, such as a cooling water circuit in the vehicle. Thereafter, the oil may be returned to the toroid-shaped space. The cooled oil may be used for cooling the retarder. As a consequence of that, a great quantity of heat is produced by the retarder during a braking process. Furthermore, the working life of the oil is influenced by high temperatures, wherein the time which pass between the exchange intervals decreases if the temperature of the oil is kept down.
FIG. 1 shows parts of a known retarder for a heavy vehicle, such as a lorry. The known retarder comprises a double rotor 1 and two stators 2, 3, which surrounds a shaft 4 to form two toroid-shaped spaces 5. The inner diameter of the respective toroid-shaped space is used as an inlet 6 and the outer diameter as an outlet 7. By having the inlet 6 in connection to the inner diameter of the torus, a delivering channel 8 has to be provided between the shaft 4 and the inlet 6. Hereby, this retarder construction takes up a great deal of space. Furthermore, it ought to be noted that when the rotor rotates, the pressure difference, which is created between the area where the inlets 6 are located and the area where the outlets 7 are located is relatively small. Consequently, no higher flow of the working medium may be obtained by this construction.